Assuming you want to make this a long-term addition to your sex life, slow and steady wins the race. You need to work the asshole over a long period of time to prepare for the whole shebang.

Start with some gentle playing like licking and rubbing with a finger, and then slowly start working the tip of a finger in a little bit. It’s a great idea to stimulate the clit while doing all of this to make it a pleasurable experience.

Keeping things slick and wet is absolutely essential. Whether you use saliva or lubricant is up to the people involved and their preference, but the wetter the better.

Eventually you can start to work a finger in deeper, but make sure you keep adding saliva or lube so it glides in smoothly and easily. Also, keep that clit happy so there’s plenty of pleasure happening as you work the asshole. Once the finger is in just leave it there and let the asshole become used to it. This is a good time to lick the clit and if you can bring her to orgasm, that’s a huge plus. Once she starts cumming with a finger deep in her ass she will begin to associate pleasure with something being up her ass.

Depending on how sensitive the person is, you can start to try adding a second finger and once that is tolerable you can start working the fingers in and out. The asshole will begin to relax and get more used to being filled. Adding a third finger will get you close to being ready for the real deal. Keep playing with that clit, either licking or rubbing, so the pleasure associates with the asshole being filled.

When you are finally used to the ass being fingered then you can begin to swap the fingers for the cock. Same rules apply here, lots of saliva/lube, start slowly, work a little in and then just stay there, no in and out, just let the asshole adjust to having a cock in it. Perhaps this is all you do for a while. Eventually when the asshole is ready, start working the cock in and out a little. From here, it’s just a matter of time before you are having full blown anal sex. Just remember to pleasure the clit so anal becomes associated with pleasure.

Best of luck and let me know if you have any further questions!